Parameters for message control already used -> long text

What causes this issue?

This entry for Message Control is already being used for another
logical message:
Message type: &v1&
Message code: &v2&
Message function: &v3&

System Response

An entry in the table for Message Control can only refer to one logical
message for an application and for a message type.
However, you are trying to use the specified application and message
type for another logical message (&v4&).
The entry is not valid.

How to fix this error?

Consider which logical message you wish to assign to the application
and message type.
This assignment must be unique.
If you do not want to change the assignment to message type &v1&
(message code &v2&, message function &v3&), you must specify another
application and/or message type for the current entry.
If you do wish to change the assignment, go back to the outbound
parameters for message type &v1& (message code &v2&, message function
&v3&) and delete the relevant entry for Message Control. You can then
enter the required logical message.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.